Papers with appropriate surface properties are printed in the heatset offset method. In this printing method the printed item is transferred from the printing plate through an offset blanket onto the paper that is fed into the heatset offset printing machine as a continuous web. The transfer of the print item with ink consisting various degrees of pigment, binder, solvent and additives in the offset method is based on ink transfer from the smooth offset blanket. The ink has been transferred to the blanket from the printing plate where the ink has adhered on the oleophilic areas after the hydrophilic areas of the printing plate have been wetted by the fountain solution. Fountain solution consists of treated tap water, puffer, wetting agents and other additives. A printing plate working with dry offset (also called waterless offset) principal can also be used. In dry offset there is a silicon layer over the printing plate that repels the ink from adhering on the non-image areas of the printing plate. After the printed item has been transferred on the paper the ink will be dried with the help of heated air in order to evaporate most of the solvent from the ink. At the same time also the fountain solution transferred from the offset blanket and most of the moisture of the paper is evaporated. After the dryer the paper web is cooled in chill rolls where after the ink film is hardened and the printed paper web can be handled further.
Certain paper properties are required for a paper used in the heatset offset printing method. A paper with appropriate properties for this printing method is known for example from the European patent 539271 and the publication GB2047568. For these papers it is characteristic that calcium carbonate CaCO3 pigment is used to optimize the porosity of the paper. The printing method itself has been described for example in the German patent publication DE 3207463.
In the earlier technique printing papers have been developed based on the fact that paper has to be porous to some degree so that it can absorb the fountain solution transferred from the offset blanket onto the paper and to make easier the removal of the moisture from the paper as the ink is dried in the dryer.